linux-kernel - Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

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Date:   Tue, 11 Aug 2020 13:46:05 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Jann Horn <jannh@...gle.com>, Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>,
        Kirill Shutemov <kirill@...temov.name>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

On Tue, Aug 11, 2020 at 1:06 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Maybe trying to reuse the page just isn't worth it?

Well, the attached patch boots, and hasn't slowed kernel compiles
down. But it may do horrible things due to leaving swap cache pages
and KSM pages to be reaped by the memory scanner, instead of being
reused.

I wouldn't notice, I have too much memory in this machine anyway.

It might have positive side effects too, of course. Not waiting for
the page lock in the page fault case could be a big win on some loads.
We do_wp_page() was one of the paths to the page lock that caused the
nasty latency spikes (I'm not sure it was a dominant one, but it was
up there).

So maybe it is worth running some test loads on. And while this patch
doesn't do it, applying this should mean that you can just revert all
the COW games entirely, and we can remove the should_force_cow_break()
from the GUP paths.

(Also - if this actually works, we can get rid of reuse_ksm_page(),
this was the only user)

                 Linus

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